The present invention relates to a directional coupler and a wireless communication device, and more particularly, to techniques for providing a flat frequency characteristic for the degree of coupling in the directional coupler.
A directional coupler (hereinafter simply referred to as the “coupler”), which enables extraction of part of power propagated on a transmission line, has become an indispensable component for designing a transmitter circuit of a variety of wireless communication devices such as portable telephones, wireless LAN communication device, communication devices of Bluetooth standard, and the like.
Specifically, a coupler forms part of an adjuster means for controlling a transmission signal to maintain at a certain level. The adjuster means comprises a power amplifier (hereinafter referred to as the “PA”) capable of controlling a gain; a coupler for detecting the level of a transmission signal; and an automatic output control circuit (hereinafter referred to as the “APC circuit”). An inputted transmission signal is amplified by the PA, and subsequently outputted through the coupler. The coupler provides the APC circuit with a monitor signal which is at a level corresponding to the level of the transmission signal outputted from the PA. The APC circuit controls the gain of the PA such that the output of the PA remains constant in accordance with the level of the monitor signal (i.e., the level of the transmission signal). Such feedback control of the PA facilitates stabilization of transmission output.
The foregoing coupler comprises a primary line and secondary line which are disposed in close proximity to each other to make electromagnetic coupling therebetween. The primary line for transmitting a transmission signal comprises an input port at one end thereof, and an output port at the other end thereof, respectively. The secondary line for detecting the level of a transmission signal comprises a coupling port at one end thereof and an isolation port at the other end thereof, respectively. Then, part of a transmission signal transmitted through the primary line is extracted by the secondary line, and is outputted to the APC circuit through the coupling port, as a monitor signal.
Main characteristics of a coupler may include insertion loss, degree of coupling, isolation, and directivity. The insertion loss is a loss caused by the coupler, and is desired to be lower. The degree of coupling indicates the ratio of power propagating in a forward direction (direction from an input port to an output port of a primary line) to power extracted to a coupling port. The isolation indicates a leakage of power propagating in a reverse direction (direction from the output port to the input port of the primary line) to the coupling port, and is desired to be higher (a smaller leakage). The directivity, in turn, indicates the difference between isolation and degree of coupling, and a better coupler is defined to exhibit a higher directivity (larger absolute value), allowing for the formation of a satisfactory APC circuit with few detection errors. For this reason, the directivity is generally required to be 20 dB or more.
On the other hand, portable terminals represented by portable telephones and smart phones employ different communication frequency bands depending on countries and regions, so that communication devices which can employ a plurality of frequency bands have been recently provided in order to flexibly accommodate such a circumstance of frequency. For example, there are a dual-band system which can employ two frequency bands; a triple band system which can employ three frequency bands; even a quad-band system which can employ four frequency bands, and so on.
The following patent documents relate to such a coupler:
Patent Document 1: JP-A-2002-280812;
Patent Document 2: JP-A-2011-61440; and
Patent Document 3: JP-A-2009-27617.